Bioaerosol particle detector

ABSTRACT

A particle detector that includes a housing defining a chamber, and an air stream injector, producing an airstream with entrained particles, in the chamber. A light source produces a light beam that intersects with and is wider than the air stream. A light detection assembly detects light generated by scattering of the light beam, by particles in the air stream. A digitizer produces a sequence of scattering digital values, each representing light detected per a first unit of time duration. Additionally, a summing assembly produces a sequence of summed scattering digital values, each equaling a sum of a sequential set of n of the digital values, and wherein successive summed digital values are offset by a the first unit of time duration and overlap by n−1 of the first units of time duration with a nearest neighbor. Finally, a detection assembly processes the summed scattering digital values to detect particles.

RELATED APPLICATIONS

This application claims benefit of provisional application U.S. Ser. No.62/620,980 filed on Jan. 23, 2018 which is incorporated by referenced asif full set forth herein.

BACKGROUND

There are many situations where it would be desirable to determinebiological aerosol levels in surrounding air. Many human, animal andplant pathogens such as viruses and bacteria travel from one host toanother either by attachment to an otherwise benign aerosol particle orthrough a purposeful aerosol lifecycle stage such as a spore. Suchbioaerosols are particularly disadvantageous in pharmaceuticalmanufacturing facilities and clean rooms of any type.

There are a limited number of ways to monitor bioaerosols. The companyProengin of Saint-Cyr-l'École, France markets a product that passes astream of sampled air through a hydrogen flame. Literature from thecompany indicates that the spectrometric signature created by oxidationof the bioaerosol can be used to detect many different types ofpathogenic organisms. However, it is likely that many carbonaceousmaterials will present very similar signatures and this approach may betoo susceptible to falsely categorize organic aerosols as biological innature to have much utility except in certain applications.

A second method that has been the subject of considerable research anddevelopment, particularly by the U.S. Department of Defense, is based onUV fluorescence. In this approach, advantage is taken of the broadcharacteristic of organism biochemicals to fluoresce when subjected toultraviolet light. Fluorescing chemicals are ubiquitous in livingorganisms, and may be compounds such as aromatic amino acids, NADH, andflavins. In these devices, a stream of air is made to pass through abeam of ultraviolet light (with the light beam usually positionedperpendicularly to the particle beam). Fluorescence emitted from anyaerosol particles is then collected optically and converted into acorresponding electronic signal by a photodetector, eitherparticle-by-particle or in an ensemble integrated mode. In both cases anelectronic signal is generated that is proportional to the localbioaerosol concentration.

This has advantages over the flame photometry approach in that it islikely to have less false positives from atmospheric organic debris,there are no consumables, and highly flammable hydrogen is not needed.In addition, differences in fluorescence intensity may help discriminateagainst some natural or manmade organic aerosol particles that are notof interest. A particle-by-particle approach is preferred because itallows a more detailed examination of aerosol composition, but theintensity of fluorescent light emitted by a single bioaerosol particleis very small, generally only 1/1000 or less the intensity of lightscattered by the particle. Hence, highly efficient methods of collectionand electro-optic conversion are required.

Preferred detection devices are the photomultiplier tube (PMT) andavalanche photodiode. The PMT has very high gain but may be somewhatbulky and fragile, while solid state devices such as the avalanchephotodiode can be quite small but have less gain and may not be capableof operation at higher temperatures. Which approach is more suitabledepends on the application.

At first glance, it would seem desirable to design the excitation opticsand air stream injector so that each particle's fluorescence signatureis maximized. This has been the approach most often used in aerosolparticle sizing devices. Particles are forced through an aperture andmade to pass through the focused beam from a laser diode. This producesa large photon pulse which is converted to an equivalent electronicpulse by the detector. These pulses are then summed, and the sampledconcentration calculated based on the known air flow. One disadvantageof this analog approach is that there is no way to know how many of thecounts are truly associated with particle transits as opposed tobackground noise pulses (such as generated internally by the detector)without halting the sampling process and measuring the background countrate. This would be extra work for an operator and force a period oftime when no sampling was being performed.

There may also be a tradeoff between maximizing the fluorescencesignature of particles that are detected, and the uniform detection of asignificant proportion of particles in the air stream. For example, theimage spot size from a solid-state laser may be made very small and thelocal light intensity very high so that particles passing through thefocal point will be intensely excited. It is exceedingly difficult,however, to focus an air stream containing 1 to 10 micron particles to acorrespondingly small cross-section. In typical embodiments either onlya fraction of the particles in the air stream pass through the laserbeam's focal point and many particles sampled by the measuring devicepass through undetected, or the particles are forced to pass throughsuch a small aperture that there is a risk of the aperture clogging overtime. In addition, the radial intensity distribution around thepropagation axis of a laser beam is typically Gaussian and particlespassing at different distances from the focal point would see varyinglevels of UV excitation, making it very difficult to quantitativelyestimate the overall aerosol sample's particle size distribution.

Essentially all bioaerosol detectors reported in the literature or soldon the market use mirror optics to collect and focus signal fluorescenceonto a photodetector. This is due in part to a lack of low-costrefractive optics at ultraviolet wavelengths and to a preference on thepart of those knowledgeable in the art to use metal reflective opticssince, to the first order, they are insensitive to the workingwavelength. Also, the curved mirrors used are focused on theintersection of the light beam and air stream, thereby maximizing thereflection of the faint fluorescent signals created. This may, howevercreate a large expense in system manufacture and maintenance, as thereflective optic components can be quite costly and are very difficultto clean without damaging the reflective surface.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

In a first separate aspect, the present invention may take the form of aparticle detector includes a housing defining a chamber and an airstream injection assembly, which produces an airstream with entrainedparticles having an average velocity within a transverse extent, in thechamber. A light source produces a light beam that intersects with andhas a minimum transverse extent equal to or greater than the airstream's transverse extent and a light detection assembly, is configuredand positioned to detect light generated by scattering of light from thelight beam, by particles in the air stream. Also, a digitizer isresponsive to the light detection assembly, to produce a sequence ofscattering digital values, each digital value representing lightdetected per a first unit of time duration. Additionally, a summingassembly produces a sequence of summed scattering digital values, eachof the summed scattering digital values equaling a sum of a sequentialset of n of the digital values, and wherein successive summed digitalvalues are offset by a the first unit of time duration and overlap byn-1 of the first units of time duration with a nearest neighbor summeddigital value. Finally, a particle detection assembly processes thesummed scattering digital values to detect particles.

In a second separate aspect, the present invention may take the form ofa method of detecting particles, which uses a particle sensor thatincludes a housing defining a chamber and an air stream injectionassembly, that produces an airstream with entrained particles having anaverage velocity within a transverse extent, in the chamber. Also, alight source produces a light beam that intersects with and has aminimum transverse extent equal to or greater than the air stream'stransverse extent. A light detection assemblyis configured andpositioned to detect light generated by scattering of light from thelight beam, by particles in the air stream and a digitizer, isresponsive to the light detection assembly, to produce a sequence ofscattering digital values, each digital value representing lightdetected per a first unit of time duration. In the method, sequentialsets of scattering digital values are summed, thereby producing asequence of summed scattering digital values, each of the summedscattering digital values equaling a sum of a sequential set of n of thedigital values, and wherein successive summed digital values are offsetby a the first unit of time duration and overlap by n-1 of the firstunits of time duration with a nearest neighbor summed digital value.Finally, the summed digital values are processed to detect particles.

In a third separate aspect, the present invention may take the form of abioaerosol detector that includes a housing, defining a chamber and anair stream injection assembly, that injects an air stream from outsidethe housing, into the chamber. Also, an ultraviolet light source,produces a light beam that intersects the air stream. A fluorescencedetection optical train, has an optical train having a long pass filter,a light detector positioned to receive and detect light from the opticaltrain and a digitizer, adapted to produce a sequence of fluorescencedigital values, each representing a fixed time duration of lightreceived by the light detector. Similarly, a light scattering detectionoptical train, has an optical train having a short pass filter, a lightdetector positioned to receive and detect light from the optical train,and a digitizer, adapted to produce a sequence of scattered lightdigital values, each representing a fixed time duration of lightreceived by the light detector. A data processing assembly performs aparticle detection routine, wherein the sequence of scattered lightdigital values is processed to detect particles and to create a particleinformation set, each time a particle is detected, including a particledetection time-period and a detected particle size, and the fluorescencedigital values corresponding to the particle detection time periods areexamined to determine a fluorescence strength for each detected particleinformation set.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following detailed descriptions.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced drawings. It isintended that the embodiments and figures disclosed herein are to beconsidered illustrative rather than restrictive.

FIG. 1 is a sectional view of bioaerosol detector according to thepresent invention.

FIG. 2 is a sectional view of the bioaerosol detector of FIG. 1, takenat a 90-degree angle to the sectional view of FIG. 1.

FIG. 3 is a block diagram describing the processing of data gatheredfrom the system of FIG. 1.

FIG. 4 is a detail view of the intersection of the light beam with thesampled air stream, occurring at the center of the sectional view ofFIG. 1.

FIG. 5 is a detail view of the intersection shown in FIG. 4, shown alongview line 5-5 of FIG. 4.

FIG. 6A is a cross-sectional view of a sampled air injection nozzle thatshifts sampled particles away from the nozzle walls.

FIG. 6B is a cross-sectional view of a second sampled air injectionnozzle having a smaller profile relative to the air injection nozzle ofFIG. 6A, while still shifting sample particles away from the nozzlewalls.

FIG. 7 is a graph of a typical cumulative distribution function forscattering photon counts per 350 μSec period.

FIG. 8 is a view of the detector of FIG. 1, partially disassembled forcleaning.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

Preferred Embodiment System Overview

Referring to FIG. 1, in a preferred embodiment of a bioaerosol detector10 (which may also be termed a “particle detector”), a housing 12defines a chamber 14. An air nozzle assembly 16, and a centrifugal airpump 18 (which may collectively be termed an “air stream injectionassembly”), create an air stream 20 having a constant flow rate. Also, alight source optical train 22 (which may be referred to simply as a“light source”), creates an ultraviolet (UV) fluorescence-stimulatinglight beam 24 at a 90-degree angle to air stream 20, such that lightbeam 24 and air stream 20 intersect orthogonally creating acylinder-shaped illuminated segment of the air stream 20 at thegeometric center of the chamber 14.

Referring now to FIG. 2, a fluorescence photon detection optical train30 and a scattering photon detection optical train 40 (which may bereferred to, singly or collectively, as a “light detection assembly”),discussed in greater detail further on, are placed on a commoncenterline that is preferably located at 90 degrees to the plane definedby the air stream 20 and light beam 24 centerlines. Fluorescencedetection train 30 is long-pass filtered to permit the detection oflight that has been emitted by fluorescent organic substances, whichwill be shifted to longer wavelengths relative to the waveband that ischaracteristic of light source optical train 22 (FIG. 1), whereasscattering detection train 40 is short-pass filtered to detectscattering from particles, which should be about 3 orders of magnitude,or more, greater in strength than the fluorescence signal. Trains 30 and40 conclude with a fluorescent photon-counting multiplier tube (PMT) 32and a scattering PMT 42, respectively.

As a non-limiting example with reference to FIG. 3, the traversal timeof a particle passing through the light beam 24 is 350 μSecs. To obtaina robust signature of the particle's travel through the beam,overlapping 350 μSec scattering and florescent transit bin counts 41 and43, respectively, are iteratively summed from 50 μSec intervals ofphoton counts (which may also be referred to as “digital values”)collected by counters 38 and 36 (which may also be referred to as“digitizers”), respectively, from PMTs 32 and 42, respectively. Althoughthe following discussion continues with 50 μSec counts, collected bycounters 38 and 36, other initial counts, typically between 25 μSec and60 μSec are used in alternative preferred embodiments. The overlapping350 μSec periods will be termed “transit bin periods,” and the summedphoton counts over these periods will be termed “transit bin counts” or“summed digital values”. The transit bin periods are incremented every50 μSecs, so that they overlap by 300 μSecs, with nearest neighbortransit bin periods. As it takes 350 μSec for a particle to traverse thelight beam, there will always be a transit bin period (having a count41) that is offset by less than 25 μSecs, for each particle traversal ofthe light beam. Particle detection 45 and particle size estimation isperformed using an array of scattering transit bin counts 41, collectedover about 50 milliseconds. For every particle detected 44, a size bin50, corresponding to a particle size range is incremented by one,thereby keeping a tally of the number of detections of particles in thatrange of sizes. Then the corresponding fluorescent transit bin countsare selected (block 46) and compared ratiometrically to net scatteringtransit bin values (block 48), with the resulting ratio (normalizedvalue of fluorescence) also assigned to a size bin 50, to keep a runningtally of the fluorescence-to-scattering ratio.

To characterize aerosols affecting human or animal health, the number ofbins may typically be from one to eight, with four bins usually beingsufficient for detailed examination without requiring excessivecomputational capabilities or creating objectionable statistical noisein the individual transit bin counts. When a potentially dangerouscondition is detected, an alarm is sounded 54. Every thirty seconds, thefinal data results are evaluated 52 and made available 54 for review bya user, including:

-   -   Concentration of aerosol particles in the airstream for each        size bin range    -   Percentage of aerosol particles that are biological in nature        for each size bin    -   Scaled fluorescence intensity for each particle size bin    -   Background fluorescence and scattering levels        Light Beam and Air Stream

Referring to FIG. 1, the UV light beam 24 is produced by a long-livedhigh-power UV LED 58, such as the NCSU033b, manufactured by NichiaCorporation of Tokushima, Japan and operating nominally at 365 nm,having a 1 mm×1 mm emitting face, over which area light is emitted veryevenly. In preferred embodiments, LED 58 emits light evenly over a facehaving an area greater than 0.5 mm². This light is focused by a lightsource optical train 22 so that the LED's emitting face is imaged onto aplane that is coincident with both the photon detection and air stream20 centerlines. A design objective for UV light intensity over the airstream 20 cross-section (See FIG. 4, 20; FIG. 5, 24) is that the lightintensity varies by 25% or less. This is difficult to achieve with asolid-state laser diode, for example, as the source is very small, maybe anisotropic in emission pattern, and is typically nonuniform inintensity over its cross-section. In a preferred embodiment, opticaltrain 22 projects light beam 24 such that it is magnified by a factor of1.5×, at the intersection with the air stream 20. In another preferredembodiment, this ratio is 2×. Typically, this ratio may range from 1.0×to 2.5×. In preferred embodiments, light beam 24 is square incross-section and has dimensions of greater than 0.75 mm by 7 0.75 mm atits intersection with air stream 20. In preferred embodiments air stream20 is round in cross-section and has a diameter of greater than 0.5 mmand less than 2 mm. In preferred embodiments, particles entrained in airstream 20 take more than 80 μSecs and 800 μSecs to transit light beam24.

Light that is not deflected by a particle traverses chamber 14 into alight dump 59 to minimize stray light reaching the detectors. Light dump59 may incorporate a fiber optic link 310 and photodetector 300, or anyalternative method for monitoring the output of UV LED 58 that does notdisturb the excitation process or the function of light dump 59.Photodetector 300 can be used to detect LED failure or to provide areference signal for ratiometrically correcting for any decay inexcitation light intensity over time.

FIGS. 4 and 5 depict detail views of the intersection of the light beam24 with the sampled air stream 20, occurring at the center of thesectional view of FIG. 1. Note that the circular side wall 63 of thenozzle 16 has not been drawn to scale with regard to the cross sectionof the light beam 24 and air stream 20. Thus, the nozzle may not bepositioned as close to the cross section of the light beam 24 and airstream 20. Rather, the scale of FIGS. 4 and 5 is for effect, tohighlight that the focused light beam 24, has preferably straight,parallel edges where the air stream 20 intersects with it and has anequal or greater minimum height and width than the air stream 20, whichtypically has a cylindrically symmetric concentration profile. Hence, ifthe focused LED image is 1.5 mm square (a 1.5× magnification ratio for a1 mm square emitting area), then the particle beam diameter must notexceed 1.5 mm. Accordingly, when this criterion is met, every particlein the air stream 20 travels the same distance across light beam 24,and, as the air stream's speed is very even over its transversedimension, spends the same dwell time in light beam 24. As the lightbeam 24 is equally bright over its transverse extent, each particle isilluminated with the same amount of light as it transits the light beam24. An equally important feature is that the air stream and light beamcross-sections at intersection are much larger than the cross-sectionsof the particles being examined, preferably on the order of 100 times ormore, making the likelihood of the air stream inlet orifice cloggingvery low. Clogging may also be prevented by the use of various upstreamfilters or screens that remove larger particles and floating debris fromsampled air.

Air stream 20 may be formed using a conventional converging nozzleassembly 16 as shown in FIGS. 1 and 2. A disadvantage of this approachis that there may be some tendency for aerosol particles to impact onand adhere to the converging portion of the nozzle. However, this isgenerally self-limiting to formation of a monolayer of aerosol debris onthat surface. An issue of more concern is that the axial velocity of airstream 20 may decrease too rapidly as the nozzle wall is approached,resulting in particles at nozzle exit having too large a range ofvelocities.

This may be counteracted by having the air stream 20 generatecentrifugal forces that move entrained particles in the desired sizerange away from the nozzle wall. To achieve a more constant air velocityin air stream 20 and to move particles away from the nozzle walls,referring to FIG. 6A, a preferred air nozzle assembly 16 includes anannular air intake 51, open to ambient air outside of detector 10, and acircular baffle structure 53 which redirects sampled air through anS-shaped radially inward directed path, causing it to accelerate as itheads towards nozzle throat 56, where it is accelerated further by thenarrowing passageway 56. A centrifugal air pump 18 (FIG. 1), is used todraw air through chamber 14, and thereby, through nozzle assembly 16.Air stream 20 emerges from air nozzle assembly 16 with a nominally flatvelocity profile across about 80% of the nozzle exit diameter and withentrained particles shifted away from the nozzle's interior walls. Thisparticle behavior is induced by making the nozzle relatively short inorder to prevent emergence of a fully developed flow profile.Centrifugal forces created by the S-shaped radially inward air flowcause entrained aerosol particles that are initially close to theexterior nozzle wall to be shifted towards the center of the flow andaway from the wall. This causes fewer particles to be lost through walladhesion prior to entering the interrogation chamber 14 as well asensuring that entrained particles reside in a more centrally locatedcross-section of the sampled air flow where the velocity is moreconstant in value. A design criterion is that air velocity (and henceparticle velocity) not deviate by more than 20% from an average valueover the area defined by the aerosol stream's maximum outside diameterprojected onto the plane defined by the excitation and detection opticsaxis'.

The Reynold's number of the air flow 20 is maintained in the laminar ortransitional range to minimize turbulence in chamber 14. In onepreferred embodiment, the sampled air flow is 1.2 liters/minute, theannular air intake 50 has a width of 1 mm and an outside diameter of 7.4mm and the nozzle exit diameter 202 is 1.5 mm (See FIG. 6A). In a secondpreferred embodiment, the air flow may range from 0.25 to 3liters/minute. To minimize radial expansion of the particle beam afterit exits the nozzle assembly 16, in any preferred embodiment the airstream 20 exits into a collection nozzle 201 (FIG. 2) that has a minimumflow diameter 203 that is substantially the same as exit diameter 202(FIG. 6A) of nozzle assembly 16.

In some situations, there may not be enough physical space toaccommodate nozzle assembly 16's S-shaped aerosol deflecting structure,or the nozzle exterior may obstruct too much signal light. Anotherpreferred embodiment to prevent aerosols from contacting the nozzleinterior walls is shown in FIG. 6B. In this approach, a cone-shapedchannel 51′ extends from the exterior of a cylindrical inlet section 65to a radially symmetric interior volume 57′. The interior volume 57′communicates with the interrogation chamber 14 through a coaxialexpansion section 61′ and cylindrical extension tube 55′. Intersectionof the cylindrically symmetric cone-shaped channel 51′ with the interiorvolume 57′ causes a lip 53′ to be formed at the interior volume 57′discharge face.

As sampled air flows radially inward through the conic channel 51′,those streamlines that enter and pass near to the lip 53′ are stronglycurved and exert a centrifugal force on entrained aerosol particles insaid stream flow. This centrifugal force urges said particles to moveaway from the wall of expansion section 61′ and cylindrical extensiontube 55′. Correspondingly, as the airflow enters the interior nozzle63′, there is an annular zone of flow adjacent to the nozzle interiorwall 56′ that is significantly reduced in respirable aerosol particlesthat are of the order of 1 micron in size, or greater.

Optionally, a second tubular sampled air inlet 49′ may be added thatcoaxially penetrates to the cylinder's interior volume 57′ from the endof the volume 57′ that is opposite the expansion section 61′. While thissecond air inlet has a negligible effect on particle contact with thenozzle interior wall, it is effective at stabilizing the air flowexiting the interior volume 57′ and entraining large particles. In theabsence of this secondary inlet flow, larger particles may tend toimpact the walls of the interior volume 57′.

The conic channel 51′ may have a width of 0.5 to 0.75 mm; the conechannel's angle relative to the axis of inlet section 65′ may range from5 to 30 degrees, and the radius of curvature of the lip 53′ shall beless than 0.5 mm. The lip diameter shall be 2 mm, the expansion angleless than 5 degrees, the extension tube diameter 2.5 mm, and the nozzleexit diameter shall be 1.5 mm. If the second air inlet is employed, thediameter may usefully range from 0.5 mm to 2. mm and the percentage ofairflow through the secondary inlet 49′ may range from 5% to 25%. In oneembodiment, dependent on the setting of the centrifugal air pump, theair stream exiting the nozzle (FIG. 6B), may have an average velocity ofabout 1500 cm/sec and a dwell time of only 100 μSec. In a secondembodiment, the average exiting air stream velocity may be 430 cm/sec.This velocity yields a 350 μSec dwell time of entrained particles in the1.5 mm square light beam 24.

As with the other aerosol deflecting structure, it is desirable tooperate either in laminar or transitional flow and to avoid turbulence,as turbulence will encourage transport of aerosol particles towards theinlet nozzle walls.

Scattering and Fluorescence Optical Pathways

As seen in FIG. 2, optical train 30 begins with a fluorescence traincollection lens 82 and optical train 40 begins with a scattering traincollection lens 92. Both lenses 82 and 92 have an object point at theintersection of air stream 20 and light beam 24 that coincides with thelens's focal length.

Collected light emerges from each collection lens 82 and 92 in anominally collimated condition, that is, the light rays are parallel tothe lens centerline. Such collimated rays are amenable to wavelengthfiltering using dichroic filters consisting of multilayer films that canbe constructed to pass either short wavelengths of light or longwavelengths (short pass and long pass filters, respectively).Accordingly, the light from lens 82 encounters long pass dichroic filter84 and the light from lens 92 encounters short pass dichroic filter 94.Both filters are aligned perpendicular to the respective lens axis. Longpass filter 84 passes the light resulting from the fluorescence offluorescent organic substances, which produces longer wavelength lightthan the light that illuminates the biological substances. The shorterwavelength light is reflected by filter 84. In similar manner, lightthat is at the wavelength emitted by light source train 22 is passed byshort pass filter 94, and longer wavelength light is reflected.

The reflected light from filters 84 and 94 is focused by lenses 82 and92, respectively to the chamber 14 center, from which point it travelsthrough the lens 92 or 82, respectively (unless the particle from whichthe light was reflected is directly in the center, in which case itpartially blocks this light). In this manner fluorescent light that isreflected from short pass filter 94 is then redirected to thefluorescent detection optical train 30 and light that is reflected fromlong pass filter 84 is redirected to scattering detection optical train40, in both cases, adding to the detected signal.

The light passing through filters 84 and 94 is focused by lenses 86 and96, respectively, to detection surfaces of photon-counting multipliertubes (PMTs) 32 or 42, respectively, where photons are counted andstored in a sequence of consecutive sampling time bins.

In a preferred embodiment, to minimize certain aberrations and reducemanufacturing cost, lenses 82, 86, 92 and 96 are of the same design.These lenses are preferably plano-convex, with the convex profile beingaspheric to maximize light capture. Further the refractive index oftheses lenses is substantially the same for the scattering andfluorescent light, and the surfaces are anti-reflection coated tominimize Fresnel reflections. Further, the antireflection coatings arevery durable and protect the underlying glass, thereby making the lensesamenable to cleaning.

To minimize the effect of refractive index variations, if there issubstantive separation of the scattering and fluorescence channel (30and 40) spectral pass bands, it is preferable to use a focal length forthe light capturing lenses 82 and 92, that is characteristic of awavelength midway between the excitation and fluorescence channel passbands. But a wavelength midway in each respective channel pass band isrecommended for the second lenses 86 and 96 that transfer signal lightto the respective photodetectors 32 or 42. Depending on the scatteringand fluorescence channel (30 and 40) pass bands, one out of a number oflow self-fluorescing optical glasses is used. In one variant this glassis fused quartz.

Starting with blocks 39 and 37, actions shown in FIG. 3 are performed bya computer, in one embodiment in the form of a dedicated computer thatis connected to the portions of the detector 10 shown in FIGS. 1 and 2,for example a laptop computer connected by a USB cable and having adisplay or an auditory warning system, or both (more broadly a “humanperceptible notification device”) for informing a user of alarmconditions (noted below) and a keyboard for user input. In analternative preferred embodiment, the computer is in the form of amicroprocessor/microcontroller with associated random-access memory andstatic memory (for storing computer instructions) which is integratedinto the portions of detector 10 shown in FIGS. 1 and 2, together with adisplay and user data input device.

Data Processing and Detection

Computation of Transit Bin Counts

As noted in the overview, sampling counters 36 and 38 deliver values ofthe number of photons (which may also be referred to as “first digitalvalues”) received by the PMTs over a collection time that typically is ⅓or less of the particle transit time. In a preferred embodiment, thesevalues are delivered to a microprocessor (not shown) for furtherprocessing, as shown in FIG. 3 and described below. In an alternativepreferred embodiment, a laptop or tablet computer is connected todetector 10, by a USB cable or other means, and has an installed programto perform the processing described below. In the embodiment in which amicroprocessor is used, a display and keyboard (not shown) may beconnected to the microprocessor, to permit a human user to input choicesand to be advised of alarm conditions and other data.

As noted previously, in one preferred embodiment it takes a particle 350μSecs to pass through the light beam 24. If consecutive 350 μSec timeintervals were used for detection, a particle that was evenly split inits transit time between time intervals would in this worst case appearat half its actual count strength in either interval. Both particlearrival time and photon emission follow Poisson statistics: To obtainmaximum accuracy in determining each particle's optical output, it isdesirable for the majority of photons emitted by any single particle toreside in a single transit time bin. To avoid these issues, thepreferable transit bin period should be about 75% or more of the actualparticle transit time, and counters 36 and 38 should accumulate countsfor coincident time periods that are not more than about ½ of thetransit bin's period. In the nonlimiting example, where the counters' 36and 38 sampling period is 50 μSecs, the sampling period and time shiftare 1/7 of the transit bin period.

Any transit bin period that exceeds the actual particle transit timewill trap all photons emitted during transit, but an excessively largetransit bin period may begin to affect the device's ability todistinguish between particles that are closely spaced in time, that is,when the sampled particle concentration is high. Accurately determiningbackground noise levels also becomes more difficult. For that reason,transit bin periods that are greater than about 2 times the actualparticle transit time are not suitable except when sampled particleconcentrations are known to be low.

Transit Bin Count Accumulation Method

In a preferred embodiment, 50 μSec samples are formed, with each samplesequentially added to each one of seven, different transit bin summinglocations 39 (collectively, a “summing assembly”), to form a part of 7different overlapping 350 μsecond sampling interval sums. Table 1illustrates this process over a span of 18 sampling periods, that is, anelapsed time of 900 μSecs. Note that the first 6 transit bin counts areshort-counts and unusable. If the total elapsed time represented by thetransit bin data array is significantly greater than the transit time,the loss of these data points is not a material issue.

TABLE 1 Example transit bin period correlation with a 350 μSec TransitBin period and a 50 μSec Sampling Bin period) Transit 50 μSecs timeincrements → bin 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1 | 1 2 |2 1 3 | 3 2 1 4 | 4 3 2 1 5↓ 5 4 3 2 1 (each Transit Time Bincorrelation is the sum of 7 small bins) 6 6 5 4 3 2 1 (Each newcorrelation is shifted 50 us in time) 7 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 97 6 5 4 3 2 1 10 7 6 5 4 3 2 1 11 7 6 5 4 3 2 1 12 7 6 5 4 3 2 1 13 7 65 4 3 2 1 14 7 6 5 4 3 2 1 15 7 6 5 4 3 2 1 16 7 6 5 4 3 2 1 17 7 6 5 43 2 1 18 7 6 5 4 3 2 1

In this method, summing locations 39 (FIG. 3) are initially set to zero.Then, over the next 350 μSec, the values from scatter channel counter 38are sequentially stored in the first 7 consecutive memory locations.After another 50 μSec, the counter 38 values are added to 7 consecutivememory locations; but the first memory location has been incremented byone location, relative to the initial step. Accordingly:

1. The first 50 μSec scattering count goes in summing locations 1, 2, 3,4, 5, 6, and 7

2. The second 50 μSec scattering count is added to summing locations 2,3, 4, 5, 6, 7, and 8

3. The third 50 us scattering count is added to summing locations 3, 4,5, 6, 7, 8, and 9

4. Further iteration.

This can be easily implemented with both counters 38 and 36. In apreferred embodiment only the scatter channel values are used to detectparticles since scattering photon count rates are typically a few ordersof magnitude higher than fluorescent photon count rates, so the photonsthat are scattered from a particle are far easier to distinguish over anoise floor than are the photons produced by fluorescence. Each transitbin that best matches the time period over which a particle traversedthe light beam will exhibit a local maximum in bin counts and thosespecific scattering and fluorescent transit bins provide the scatteringand fluorescence information necessary to characterize the sampledaerosol environment. Using the information contained in these transittime bin arrays, a number of useful factors can be determined from thefluorescent photon and scattering photon signatures recorded by PMTs 32and 42 including most importantly, net scattering and fluorescencephoton counts on a particle-by-particle basis. Various peak-findingalgorithms can be employed to search out and identify these localmaxima, and standard statistical tests can be used to ensure that theyare not random peaks attributable to background noise.

In a preferred embodiment, the processes of fluorescent and scatteringtransit bin accumulation are automated and implemented as direct memoryaccess (DMA) procedures. This saves processor time, which can then beused to analyze long arrays of transit bin counts. Also, as the processis firmware implemented, it can be changed in firmware, yielding greaterflexibility in development and use.

Algorithms for Determining Mean Noise and Standard Deviation

A significant advantage of this invention is the ability to calculatesystem background noise in real-time. Conventional analog pulse methodswhere each received electronic pulse is assumed to represent a particletransit are fundamentally flawed in that many of the received counts maybe attributable to system noise. The only way to determine thebackground count in such analog systems is to cease sampling anddetermine the background count rate empirically. This is notsatisfactory as an important event may occur during this dead time.Here, background noise calculations are performed for both channelswhile peak (particle crossing) detection is performed using the scatterchannel transit bin counts 41. Background noise count rates areimportant since they can be subtracted from gross count rates todetermine net photon counts attributable to each particle's scatteringand fluorescence transit signatures. Also, they are a factor in settinga threshold for particle detection.

Statistical methods are used to accomplish this efficiently. Samplingcounts attributable to background noise sources are recorded by thescattering and fluorescence PMTs 42 and 32 contemporaneously with countsattributable to particle transits. While particle transits produce shortbursts of photon counts, noise counts are registered at a much loweraverage rate and are randomly distributed in time. Background counts arereasonably well characterized by Poisson statistics. At higherbackground noise levels, the distribution of counts around the meantends towards a Gaussian distribution. If, as depicted in FIG. 7, acumulative distribution of transit bin data is plotted as a function oftransit bin count, a classic S-shaped curve is found that is typical ofa Poisson or Gaussian process. with the plateau 76 above the meancorresponding to the transit bin count range above which backgroundnoise events are rare. At higher value transit bin counts there is asecondary plateau 78 in the cumulative distribution caused by thepassage of highly scattering or fluorescing particles through theexcitation beam. This feature occurs further to the right in a plot ofcumulative bin statistics, after the noise background has reached itsseparate cumulative plateau 76. That is, there are two plateaus 76 and78, in the cumulative distribution as shown in FIG. 7.

Therefore, to determine the average background count rate in real-time,the point on the curve where the cumulative distribution's value isone-half the noise background plateau's value is determined. This is bydefinition the mean background count rate. The same background protocolis used to calculate background levels in the fluorescence channel. Oncethe mean values are determined, it is straightforward to estimate thestandard deviations of the background counts. This in turn provides acriterion for the scattering channel for detecting particle transits,while avoiding false alarms caused by natural variations in backgroundnoise. As a practical matter even if the two plateaus are not wellseparated, calculation of the mean background and standard deviation arenot affected in a material way.

In a preferred embodiment, the particle detection threshold is set at 4standard deviations above the scattering channel's mean background noiselevel. The scattering channel transit bin data points corresponding topeak detections identify all particle passages, both biological andnon-biological, since all particles scatter light. Net counts for allparticle transits are determined by subtracting the fluorescent andscattering channel mean background counts from the measured grosstransit bin particle counts for the transit bin data points thatcorrespond to peak detections.

Categorizing and Processing Detections

In a preferred embodiment, particle size is estimated based onelectromagnetic scattering theory. The intensity of light scattered bymicron-size particles when exposed to collimated UV light scalesapproximately as the square of particle size. Hence, particle size isapproximately proportional to the square-root of the net scatteringphotons produced when a particle traverses the excitation beam. In analternative embodiment, actual empirical tests of scattered photon countversus particle size are performed to calibrate the particle sizeestimation. Armed with particle size information (part of a “particleinformation set”), particles can be categorized into a set of size bins50.

The fluorescence counts corresponding in time to each particlescattering event are examined and if these counts are determined to bestatistically significant compared to the fluorescence channelbackground noise, the background counts for the fluorescence channelover the transit period are subtracted from the gross transit counts andthis information is also stored for each size category as the netfluorescence for that particle.

In a preferred embodiment, the net fluorescence counts for particles ineach size bin are ratiometrically normalized to the corresponding netscattering counts. That is, the net fluorescence counts for thebioaerosol particles are divided by the net scattering counts. In thisway, the fluorescence emission intensity is measured relative to thescattering intensity. To a first order, this method eliminatesdependence on excitation intensity since both types of photon emissionhave approximately the same dependence on particle size. For thecritical respirable particle size range of 1-10 microns, robust resultsare achieved by segregating this range into four or more size bins 50.In a preferred embodiment, the bins are defined as ranges of 1 to 2microns; 2 to 4 microns; 4 to 7 microns; and 7 to 10 microns. However,the number of bins and the size ranges may need to be adjusted to fitdifferent applications.

If this normalization is not done, the fluorescence bins will be tooheavily weighted towards counts from large particles. For example,consider the case where the second bin size range above contains twospherical particles, a two-micron diameter particle and a four-micronparticle, and where both particles have a fluorescence emission rate perunit area of 1 count per square micron. Then the cumulative fluorescencecounts from the two particles in that bin can be calculated to be 20πcounts. A casual observer might then calculate, based on an averageparticle size of 3 microns, that the fluorescence emission rate per unitarea is about 2.22 counts per square micron for the bin, whereas it isonly 1 count per square micron, resulting in an estimate that is 2.22times larger than actual, and a value that mischaracterizes the bin'sbiological character significantly.

By using the approach described herein, the user has access to theinformation needed to develop a detailed profile of the surroundingenvironmental aerosol milieu, including:

-   -   Number and size of all aerosol particles    -   Percentage of aerosol particles that are biological in nature,        and their size    -   Scaled fluorescence intensity of each bioaerosol particle size        bin    -   Background fluorescence and scattering levels

In a preferred embodiment, a rolling average of fluorescent andscattering particle concentrations and fluorescence intensity ismaintained over an averaging period of 10 to 60 seconds for each sizebin as a baseline or reference for alarm criteria. In a preferredembodiment, if there is a change over the length of one averaging periodin the bioaerosol level that is four standard deviations greater thanthe rolling average, an alarm condition is entered. If the bioaerosollevel exceeds a set high value, such as 1000 particles per liter of air,or the fluorescence intensity of a bin exceeds a specified level, thenan alarm condition also is entered.

Alternatively, if an aerosol component suddenly appears that has anextremely high ratiometric fluorescence intensity (the ratio offluorescence signal to scattering signal), this may indicate thepresence of a benign manmade agent such as a paper whitening chemical ora natural fluorophore such as a polycyclic aromatic hydrocarbon. Thecondition may be ignored, or the alarm downgraded to a lower risk level,for example if the ratiometric fluorescence intensity in one or moresize bins exceeds a benign fluorescence detection threshold, set at 0.8,or in an alternative embodiment 0.5 of the signal expected from such areference benign fluorescent aerosol substance. In an alternativeembodiment, the threshold for benign fluorescence detection is setrelative to the highest ratiometric fluorescence intensity that could beexpected from a threatening fluorescent aerosol substance, for exampleat twice this level. In another preferred embodiment, the benignfluorescence detection threshold is set at a point between the highestexpected threatening ratiometric fluorescence intensity and the lowestexpected benign ratiometric fluorescence intensity, for example at themidway point between these levels. The ability to monitor not only thebioaerosol concentration but also the intensity of the bioaerosol'sfluorescence allows a more nuanced approach to dealing with changes inthe suspected bioaerosol background.

In another preferred embodiment, when a sudden large increase in thebioaerosol concentration is detected that causes an alarm, the rollingaverages may be frozen at pre-alarm values for a pre-selected timeperiod, such as 2 to 10 minutes so as to prevent this situation fromabnormally affecting the rolling average baseline.

The above described method yields great advantage over bioaerosoldetectors that use only a count of statistically significant fluorescentsignal events to report the presence of a bioaerosol particle and whichdo not store the actual florescence channel signal strength level duringdetected particle beam crossings. In the approach described above, thefluorescence intensity may yield information about the type of particlebeing examined. Also, in systems in which particles are not assigned tosize ranges, useful information is lost as to the particle size rangecontaining the largest number of biological aerosol particles.

Noise Processing

In a preferred embodiment, the nominal background signal (“backgroundsignal” may also be termed “noise” “background noise” or “backgroundlevel”) levels in each channel and their standard deviations are alsotracked. These variables are used in three different ways. First, theyprovide an ongoing measure of surface contamination and are compared toa caution threshold referenced to a baseline noise level, at the sametime as alarm criteria are evaluated to provide a warning of when theinterior surfaces should be cleaned to reduce background noise to asustainable level. In embodiments, this test evaluates noise levels overan extended period of time, more than an hour, such as a day, week ormonth as surface contamination tends to build up slowly and is generallya constant and unvarying presence. A baseline noise level may be formedduring the first period of operation of device 10, when first put intoservice or after the most recent cleaning of interior surfaces, and inembodiments, the threshold is formed by multiplying this baseline by aconstant (which may also be termed a “background noise increasethreshold”). Correction for any other factor that affects noise levels,such as average temperature, is performed in embodiments. Second,background levels may suddenly rise if the sampled air is filled withaerosol particles below the size resolution limit of the system. Thatis, the sudden presence of a large number of small sampled particles canproduce a high background level. A rapid increase in background countscan therefore signal that a high density of small particles has beenencountered, and this information may be transmitted to the user. Thisallows the presence of particles that are smaller than the system sizeresolution limit to be detected in a qualitative or ratiometric manner.Independent of any specific size resolution limit, if the inferredfluorescent or scattering channel background noise increases by a factorof 2 times or more over one averaging period, or by a statisticallyunlikely amount, such as, in embodiments, either three or four standarddeviations, it is presumed that this is due to a significant increase insmall particle concentration, and personnel are notified by a display ora sound. In alternative preferred embodiments, the test for notifyingpersonnel is calibrated by the absolute level of the initial backgroundnoise and environmental conditions, which are either sensed by othersystems and communicated to device 10, or entered by personnel.

Detecting Photon Flood at the Scattering Channel PMT

Referring to FIG. 3, At the output of scatter channel PMT 42, eachphoton encounter appears as a pulse, which is then registered by counter38. But if a second photon encounter begins before the pulse from afirst photon encounter has transitioned back to zero, then the twopulses appear as one, and only one photon encounter is detected andcounted by counter 38. If a constant stream of photons is beingencountered by scattering PMT 42, this results in a DC signal at itsoutput, and counter 38 will only register a detection when there is agap in the photon stream. To avoid being misled by this condition, a DCdetector 88 is placed in parallel with counter 38. When a continuous DChigh-state signal is detected, a fault alarm warning is relayed to theuser.

Ease of Cleaning

Referring to FIG. 8, in the detector 10 described here, each opticschannel 30 and 40 can be mounted as a separate sealed assembly that canbe removed as a single unit from the central module that contains theparticle and light beam forming components (FIGS. 1-3). Although notshown, in one embodiment each channel is bolted onto the rest of thehousing using the bolt holes shown. The planar faces of lenses 82 and 92are the most likely surfaces to be affected by fouling, and byorganizing the optical system's physical structure in this way, each ofthe two critical light-capturing lens faces may be easily cleaned withminimal risk of scratches and residual surface debris, similar tocleaning the front lens face on a camera. This is a very critical issuefor applications where the devices may be exposed to dusty inorganicaerosols such as clay and sand. Even if precautions are taken tominimize dust accumulation on the light capturing optical faces therewill typically be some aerosols that reach these surfaces, such asduring startup and shutdown of the sampling airflow when the airflowsmay become temporarily turbulent.

While a number of exemplary aspects and embodiments have been discussedabove, those possessed of skill in the art will recognize certainmodifications, permutations, additions and sub-combinations thereof. Itis therefore intended that the following appended claims, and claimshereafter introduced, are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

The invention claimed is:
 1. A particle detector, comprising: a. ahousing defining a chamber; b. an air stream injection assembly,producing an airstream with entrained particles having an averagevelocity within a transverse extent, in said chamber; c. a light source,producing a light beam that intersects with said airstream and at thisintersection has transverse extent equal to or greater than said airstream's transverse extent; d. a light detection assembly, configuredand positioned to detect light generated by scattering of light fromsaid light beam, by particles in said air stream; e. a digitizer,responsive to said light detection assembly, to produce a sequence ofscattering digital values, each digital value representing lightdetected per a first unit of time duration; f. a summing assembly,producing a sequence of summed scattering digital values, each saidsummed scattering digital value equaling a sum of a sequential set of nof said digital values, which represents a time duration equal to theaverage travel time of an entrained particle though said light beam, andwherein each sequential pair of summed scattering digital values aremutually offset by a said first unit of time duration and mutuallyoverlap by n−1 of said first units of time duration; and g. a particledetection assembly, which processes said summed scattering digitalvalues to detect particles.
 2. The particle detector of claim 1, furtherbeing a biofluorescence detector and wherein: a. said light source is anultraviolet light source; b. said light detection assembly is ascattered light detection assembly, including a short pass filter, andwhich detects shorter wavelength light; c. further comprising afluorescence light detection assembly, including a long wavelengthfilter, and which detects longer wavelength light indicative of lightthat has been created by fluorescence from said particles in said airstream; and d. a digitizer, responsive to said light detection assembly,to produce a sequence of fluorescence digital values, each fluorescencedigital value representing light detected per said first unit of timeduration.
 3. The particle detector of claim 2, wherein said summingassembly also produces a sequence of summed fluorescence digital values,each summed fluorescence digital value equaling a sum of a sequentialset of n of said fluorescence digital values, and wherein successivesummed fluorescence digital values are offset by a said first unit oftime duration and overlap by n−1 of said first units of time durationwith a nearest neighbor summed fluorescence digital value.
 4. Theparticle detector of claim 1, wherein said processing said summedscattering digital values includes determining a detection threshold byanalyzing said summed scattering digital values and comparing each saidsummed scattering digital values to said threshold.
 5. The particledetector of claim 4, wherein said analyzing includes analyzing acumulative distribution function of said summed scattering digitalvalues.
 6. The particle detector of claim 5, wherein said processingincludes detecting a first plateau in said cumulative distributionfunction of said summed scattering digital values.
 7. The particledetector of claim 4, wherein said threshold is set to have a particlefalse positive detection rate, of less than a maximum specified rate. 8.The bioaerosol detector of claim 4, wherein sequential sets of saidscattered light digital values are summed to create summed scatteredlight digital values and sequential sets of fluorescence digital valuesare summed to create summed fluorescence digital values.
 9. Thebioaerosol detector of claim 8, wherein net scattering digital valuesand net fluorescence digital values are determined for each particle bysubtracting the average scattering and fluorescence background noisevalues from the summed scattering and fluorescence digital values foreach particle detection time period.
 10. The particle detector of claim1, wherein said light detection assembly includes a photon multipliertube and said scattering digital values each represent a photon countper a said first unit of time duration.
 11. A method of detectingparticles, comprising: a. providing a particle sensor, including: i. ahousing defining a chamber; ii. an air stream injection assembly,producing an airstream with entrained particles having an averagevelocity within a transverse extent, in said chamber; iii. a lightsource, producing a light beam that intersects with said airstream andat this intersection has a transverse extent equal to or greater thansaid air stream's transverse extent; iv. a light detection assembly,configured and positioned to detect light generated by scattering oflight from said light beam, by particles in said air stream; v. adigitizer, responsive to said light detection assembly, to produce asequence of scattering digital values, each digital value representinglight detected per a first unit of time duration; b. summing sequentialsets of scattering digital values, thereby producing a sequence ofsummed scattering digital values, each said summed scattering digitalvalue equaling a sum of a sequential set of n of said digital values,which represents a time duration equal to the average travel time of anentrained particle though said light beam, and wherein each sequentialpair of summed scattering digital values are mutually offset by a saidfirst unit of time duration and mutually overlap by n−1 of said firstunits of time duration; and c. processing said summed digital values todetect particles.
 12. The method of claim 11, further being a method ofevaluating biofluorescence, and wherein: a. said light source is anultraviolet light source; b. said light detection assembly is ascattered light detection assembly, including a short pass filter, andwhich detects shorter wavelength light; c. further comprising afluorescence light detection assembly, including a long wavelengthfilter, and which detects longer wavelength light indicative of lightthat has been created by fluorescence from said particles in said airstream; and d. a digitizer, responsive to said light detection assembly,to produce a sequence of fluorescence digital values, each fluorescencedigital value representing light detected per said first unit of timeduration.
 13. The method of claim 12, wherein said summing assembly alsoproduces a sequence of summed fluorescence digital values, each summedfluorescence digital value equaling a sum of a sequential set of n ofsaid fluorescence digital values, and wherein successive summedfluorescence digital values are offset by a said first unit of timeduration and overlap by n−1 of said first units of time duration with anearest neighbor summed fluorescence digital value.
 14. The method ofclaim 12, wherein said particles are detected by comparing said summeddigital values to a threshold and said threshold is set to have aparticle false positive detection rate, of less than a maximum specifiedrate.
 15. The method of claim 11, wherein said processing said summedscattering digital values includes determining a detection threshold byanalyzing said summed scattering digital values and comparing each saidsummed scattering digital values to said threshold.
 16. The method ofclaim 15, wherein said analyzing includes analyzing a cumulativedistribution function of said summed scattering digital values.
 17. Themethod of claim 16, wherein said processing includes detecting a firstplateau in said cumulative distribution function of said summedscattering digital values.